Extreme Green: Earth Recycles 2.5-Billion-Year-Old Ocean Crust

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The remains of a real-life journey to the center of the Earth are
preserved in a South Pacific volcano, a new study suggests.

The lava that erupted from the Cook Islands volcano, called
Mangaia, contains a few tiny grains of sulfide, a mineral, with a
peculiar ratio of sulfur isotopes, according to research
published in today's (April 24) issue of the journal Nature. The
unusual ratio could only have formed before
oxygen-breathing life appeared on Earth 2.45 billion years
ago. Isotopes are versions of elements with different numbers of
neutrons, giving them differing weights.

The study researchers think the sulfide formed at Earth's surface
ages ago in ancient oceanic crust, and then sank deep into
Earth's
mantle, likely all the way to the core-mantle boundary, 1,865
miles (3,000 kilometers) below the surface. Some billions of
years later, a plume of hot material rising from above the core
ferried the sulfide skyward, until it escaped through Mangaia
about 20 million years ago.

The findings are direct evidence that oceanic crust was recycled
in the mantle, Cabral said. Scientists are pretty confident that
over millions of years, giant convection cells churned the stiff
rock inside the mantle, the layer between Earth's thin crust and
iron core. Convection could also recycle crust that disappears
into the mantle via
subduction zones, the plate boundaries where one tectonic
plate dives underneath another. Images derived from seismic
waves, which change speed when passing through cold or hot
materials, have revealed
possible oceanic crust piled near the core.

"The fact we have a time constraint is great for figuring out
exactly how vigorous convection is in the mantle, and how
extensive it is, " Cabral told OurAmazingPlanet. "It's very
exciting, and I'm looking forward to seeing what models come out
of it. If there are areas where this material can sit around for
a couple billion years, that's something really important."

The sulfur isotopes pin a minimum age on the source of Mangaia's
lava, so the lava could be even older than 2.45 billion years.
Before that time, there was no protective ozone layer on Earth,
because there was little oxygen in the atmosphere. Ultraviolet
radiation from the sun strongly influenced sulfur chemistry in
the atmosphere, leaving a distinctive chemical signature in the
rocks. When oxygen-breathing life appeared, sulfur chemistry
shifted dramatically.

"I think this is another really strong piece of evidence that
material from the surface of the Earth gets subducted and
transported to the mantle and ultimately returns in these mantle
plumes," said William White, a geochemist at Cornell University
who wasn't involved in the study. "My suspicion is that this has
been tucked away at the base of the mantle for 2.5 billion years
or so."

"Stuff from the very deepest mantle is forming these ocean island
volcanoes, and I think the real takeaway is the fact that there's
an intimate connection between surface material and the deep
mantle," White said. "Some of the things down there were once at
the surface of the Earth."

Time capsule

Cabral explained that volcanic islands each have unique chemical
signatures — think of them as having flavors. Scientists are
still sorting out the reasons for the different flavors. Some
islands may come from subducted oceanic crust, while others could
be sediments, or even
fragments of continents. But there are odd geochemical
signals, such as the sulfur isotopes Cabral and her co-authors
found, that hint at

stranger stuff going on in the mantle.

"Some of the chemistry in these mantle-derived lavas — some of
the things we don't understand — might reflect some of the
surface history of the Earth," White told OurAmazingPlanet.

The findings help confirm that the mantle can store very old
crust for billions of years. In this case, it gave geochemists a
window into Earth's early history. Other odd rock chemistry has
led scientists to conclude that there
may be even older rocks in the mantle, from before 4 billion
years ago, said Steve Shirey, a geochemist at the Carnegie
Institute for Science in Washington, D.C., who was not involved
in the study. (The Earth is 4.54 billion years old.)

"We don't know how we go from what we see on the ocean floor to
what we see at the depths,” Shirey said. “At this stage, a lot of
things are possible.”